Chemical Engineering May 2021 - 28

Facts At Your Fingertips
Evaporator Equipment Types
Department Editor: Scott Jenkins
ndustrial evaporator heat-transfer
surfaces are either tubular or flat
plates, packaged into a variety of
evaporator types, as outlined here.
I
Batch evaporators
Batch evaporators are vessels with a
heating jacket or internal coil, an overhead
condenser, a condensate receiver,
and usually, a vacuum source.
After feed is charged into the vessel,
heat is applied and evaporated vapor
is condensed overhead, while the
contents of the vessel decrease in
volume and increase in concentration
of non-volatile materials.
Short-tube vertical evaporators
A shell-and-tube heat exchanger is
situated inside the evaporator vessel,
near the bottom. The heat exchanger,
called the calandria, contains an
open area at the center, known as
the downtake. Process fluid circulates
upward through the calandria
tubes, against condensing steam
on the shell side. The vapor formed
travels to the top of the evaporator,
where entrained liquid droplets coalesce
on a mesh pad and fall back
into the boiling liquid. Meanwhile, the
liquid emerging from the calandria
tubes travels downward through the
downtake, then back up through the
tubes for a subsequent pass. Circulation
occurs by natural convection.
Long-tube vertical evaporators
These evaporators operate with a thin
film of liquid on the heat-transfer surface.
As evaporation takes place, vapor
fills the core of the flow channel,
which thins and accelerates the film.
Forced-circulation evaporators
With this type, process fluid circulates
from the vapor-liquid separator (flash
chamber) through the heat exchanger
and back. The orifice plate applies
enough backpressure to prevent boiling
in the heat exchanger. Only sensible
heat (no latent heat) is transferred
in the heat exchanger, and the process
liquid exits the heat exchanger at
a temperature above the boiling point
at the prevailing pressure in the flash
28
Type
TABLE 1. ADVANTAGES AND DISADVANTAGES OF DIFFERENT EVAPORATOR TYPES
Disadvantages
Advantages
Batch
evaporator
Short-tube
vertical
evaporators
* Simplicity
* Flexibility
* Relatively low cost
* Ability to handle feeds containing
undissolved solids
* Because the tubes have large diameters
and short lengths, they are easily cleaned
(well suited for materials requiring mechanical
descaling)
* Low headroom requirement
* Proven designs
* Relatively low cost
Long-tube
vertical
evaporators
* Product is a thin film (not filling entire tube
volume), so liquid holdup and residence
time, are low (lower heat exposure)
* Low cost per unit area
* Simple construction and small floor footprint
* Ability to handle foamy liquids
* Ability to handle corrosive process streams
Forcedcirculation
evaporators
Plate
evaporators
Chosen
when a film evaporator will not work.
Such applications include viscous liquids,
because these kinds of liquids do not form
films easily
* Corrugations and tortuous flow path lead
to turbulent flow at low Reynolds numbers
* The resulting high heat-transfer coefficients
allow a given evaporation rate to be
reached at a lower ∆T
* Low holdup and short residence times, so
good for heat-sensitive products
* High fluid velocity results in better heat
transfer and a lower rate of fouling compared
to tubular evaporators
* Plate packs are easily disassembled for
inspection and cleaning
Agitated
thin-film
evaporators
Residence time is short (a few seconds), and
residence-time distribution is narrow
chamber. The flashed vapor is directed
to an overhead condenser, usually
via a mesh pad to recover entrained
liquid droplets. Meanwhile, the concentrated
liquid makes another pass
through the
heat
exchanger,
Plate evaporators
This type has a series of corrugated
metal plates, separated by gaskets
around the periphery of each plate.
The plates are pressed together
to form a series of flow channels.
Steam and process fluid are directed
to alternate channels, and heat is
transferred across each plate from
the steam side to the process side.
Slotted openings and seals direct the
various streams. Baffles create a tortuous
flow path for the fluid, increaswith
some
new feed added, and some
concentrate removed as product.
Unlike the other types of evaporators discussed,
the agitated TFE is typically used
with organic, rather than aqueous, systems
ing local velocity, and in turn, heattransfer
efficiency.
Agitated thin-film evaporators
These evaporators form films mechanically,
using a rotating blade near
(or contacting) the heat-transfer surface.
The device has a mechanical rotor
concentric to a cylindrical, jacketed
body. Feed entering the top is distributed
evenly by the rotor, then the fluid
spirals down the heated wall. Bow
waves generated by the rotor blades
cause high turbulence, so heat-transfer
coefficients are high. Concentrate exits
the bottom. Vapor travels from the
heated surface to an internal condenser,
concentric to the evaporator body,
and the resulting condensate (distillate)
proceeds downward to exit.
n
Editor's note: This content is adapted from: Gabelman, A., Evaporators:
Design Concepts and Equipment Selection, Chem. Eng., January
2020, pp. 27-38.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2021
* Less efficient than film evaporators
* Lower heat-transfer coefficients than thinfilm,
so higher cost per unit area
* Cost is higher because a large circulation
pump and piping are required
* A propensity for air leakage at higher
temperatures
* Not economical for high capacities
* Low heat-transfer coefficients
* Low heat-transfer area per unit volume
* Lower productivity versus continuous
* Not suitable for heat-sensitive products
because of the extended residence time
* Low heat-transfer coefficients
* Low heat-transfer area per unit volume
* High floor-space requirement and weight
* High holdup of process material
* Use with corrosive materials is expensive,
because evaporator bodies would need to
be made from corrosion-resistant alloys
High-viscosity fluids (>300-400 cP) are not
suitable because film formation is difficult
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